In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities when compared with metals, coupled with the fact that the engineering limits of performance of many modern components and systems are now gated by these properties in conventionally employed materials. Examples of areas for such prospective use include engine components, heat exchangers, cutting tools, bearings and wear surfaces, pumps, and marine hardware.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse kinds of filler or preform materials such as particulates fibers, rods or the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications and one patent assigned to the same owner as this application overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. Thus, commonly owned U.S. Pat. No. 4,713,360 which issued on Dec. 15, 1987 and was based on U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which was a continuation-in-part of application Ser. No. 776,964, filed Sept. 17, 1985, which was a continuation-in-part of application Ser. No. 705,787, filed Feb. 26, 1985, which was a continuation-in-part of application Ser. No. 591,392, filed Mar. 16, 1984, all in the names of Marc S. Newkirk et al and entitled NOVEL CERAMIC MATERIALS AND METHODS OF MAKING THE SAME, disclose generically the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a Parent metal precursor. Molten metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation product toward the oxidant and further oxidizes, thereby continuously developing a ceramic polycrystalline body. The process may be enhanced by the use of an alloyed dopant, such as is used in the case of oxidizing aluminum in air to form .alpha.-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as disclosed in commonly owned U.S. application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of application Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of application Ser. No. 632,636, filed July 20, 1984, entitled METHODS OF MAKING SELF-SUPPORTING CERAMIC MATERIALS, all in the names of Marc S. Newkirk et al.
This oxidation phenomenon was utilized in producing composite ceramic bodies as described in commonly owned and copending U.S. Pat. application Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of application Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al and both entitled COMPOSITE CERAMIC ARTICLES AND METHODS OF MAKING SAME. These applications disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal precursor into a permeable mass of filler, thereby embedding the filler within a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in the commonly owned and copending U.S. application Ser. No. 861,025, filed May 8, 1986, entitled SHAPED CERAMIC COMPOSITES AND METHODS OF MAKING THE SAME and in the names of Mark S. Newkirk et al. In accordance with the method of this invention, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of the original parent metal body are disclosed in commonly owned and copending U.S. application Ser. No. 823,542, filed Jan. 27, 1986, in the names of Mark S. Newkirk et al and entitled INVERSE SHAPE REPLICATION METHOD OF MAKING CERAMIC COMPOSITE ARTICLES AND ARTICLES OBTAINED THEREBY.
The entire disclosures of all of the foregoing commonly owned patent applications and Patent are expressly incorporated herein by reference.
A key element in using the methods of the above-mentioned commonly owned copending applications and Patent to produce net or near net shape ceramic body, including composite bodies which retain essentially the original shape and dimensions of the filler or preform, is to minimize or inhibit ceramic matrix overgrowth of defined surface boundaries. Overgrowth of the surface boundaries can be substantially prevented by controlling the infiltration of the polycrystalline ceramic matrix to any defined surface boundaries, which may be accomplished such as by using a predetermined quantity of parent metal, establishing within the preform favorable oxidation kinetics, exhausting the oxidizing atmosphere or lowering the reaction temperature. Any of these steps may require close control or vigilance to obtain essentially no polycrystalline overgrowth of any defined surface boundary, and still may not produce the most desirable net or near net shape, or may require additional machining or finishing.
The present invention provides means for reliably establishing a boundary or substantially preventing overgrowth of the developing oxidation reaction product which is desirable in forming net shapes particularly with larger, single-piece bodies or bodies with complicated geometry.